This application claims the priority of Japanese Patent Applications Nos. 2001-299226 and 2001-299227 and 2001-301804 filed on Sep. 28, 2001 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electronic endoscope having an optical zoom mechanism, and more particularly, to image processing by an electronic endoscope in which a movable lens is driven to bring a tip of a scope close to an object to be observed, and to take an enlarged image.
2. Description of the Related Art
An electronic endoscope takes an image, by an image pickup device such as a CCD (Charge Coupled Device), of an object to be observed, captured via an objective optical system by applying illumination light, and displays the image of the object to be observed on a monitor or the like. Recently, in the electronic endoscope of this type, a movable lens (varifocal type) has been incorporated into the objective optical system, and the movable lens is moved back and forth by a zoom mechanism to optically enlarge the image of the object to be observed. The enlarged image is processed and displayed on the monitor or the like, thus allowing satisfactory observation of details of a site to be observed.
However, in the electronic endoscope having an optical zoom mechanism, there is a problem that varying light distribution prevents an image with even brightness from being obtained in close up photography for obtaining an enlarged image. FIG. 15 shows a state in which a tip of a scope is brought close to the object to be observed, and as shown in this drawing, there are provided, at a tip of a scope 2 for taking an image of an object to be observed 1, illumination windows (lenses) 4a, 4b for applying light guided by a light guide 3, and an observation window (lens) 6 of an objective optical system 5.
At a distance where no zoom function is used, lights S1, S2 from the illumination windows 4a, 4b overlap and are applied to the object to be observed 1, but when the tip 2 is set at a close up distance Da shown in FIG. 15, the lights S1, S2 from the illumination windows 4a, 4b do not overlap, and an area z to which no light is directly applied (a dotted area) appears in the object to be observed 1 as shown in FIG. 16.
In illumination areas of the lights S1, S2, intensity of the light becomes lower from a center of a light spot toward a periphery, and also in the area z, light amount becomes smaller at a position farther from the illumination positions, causing light distribution on the object to be observed. Further, when the zoom function is used, a focusing distance changes depending on power, thus intensity of the illumination light on the object to be observed 1 changes, and the light distribution also changes. In the close up photography using the zoom mechanism, unlike normal photography, there is a problem that the light distribution has a large influence, and an image with even brightness is hard to obtain.
Further, when the movable lens is driven, for example, to an enlargement end (Near end) to bring the tip of a scope 2 extremely close to the object to be observed 1 as shown in FIG. 15, both lights S1, S2 scatter inside the object to be observed 1 (for example, in a mucosa layer) in the area z of the object to be observed 1, to which no light is directly applied in FIG. 16. Thus, there is a problem that an image with redness is formed in taking an image of an object to be observed in vivo by the electronic endoscope.